This invention relates in general to submersible pumps and in particular to a system for monitoring at the surface the pressure and temperature in the pump motor environment.
The submersible pump installations concerned herein include a large electric motor located in the well. The electric motor receives three-phase power over a power cable from the surface with voltages phase-to-phase being commonly 480 volts or more. The electric motor drives a centrifugal pump to pump well fluid to the surface.
It is important to be continuously aware at the surface of the downhole operating conditions. The pressure of the lubricant in the motor is the same as the well fluid pressure, and provides an indication of whether or not the pump is operating efficiently. Temperature also provides an indication of whether or not the motor is overheating, which might possibly cause early failure. U.S. Pat. No. 3,340,500 issued to C. A. Boyd et al discloses a system for monitoring pressure using the power cables as a linkage between downhole sensors and uphole receiving units. The Boyd patent superimposes a DC level on the AC power conductors, with changes in the DC level being proportional to the physical parameter sensed. There are other later patents that also utilize the principle of passing DC current over AC lines and through a sensor to provide a resistance change that is indicated at the surface.
Improvements are desirable because of the extreme conditions in the well. A pump and any downhole sensing and measuring equipment normally remains in the well for a year and a half or more before being pulled to the surface for maintenance. The temperature is often 200.degree. F. and higher. The voltage and current being supplied to the motor are also at high levels.